1. Field of the Invention
This invention relates to an apparatus and method of detecting a perforation or other indicium on a web or the like for providing signals to initiate an operation, such as for example, timing of an exposure upon a photosensitive web in an electrophotographic reproduction apparatus.
2. Brief Description of the Prior Art
Modern high speed electrophotographic copiers or reproduction machines may use flesh lamps to illuminate, i.e., expose, the original being copied. Use of this type of lamp is one way of providing the necessary exposure speed for very high speed copying. in these arrangements, the entire original is illuminated by the flash lamps providing what is known to the art as full frame exposure of the photoconductor as the photoconductor in the form of a web moves continuously during the exposure operation. Prior to the exposure a uniform primary electrostatic charge is formed on the photoconductor. Subsequent to the exposure the photoconductor, with the charge thereon modulated with the image of the original, is developed to form a visible image and this image may be transferred to a receiver sheet.
To control the various operations in this apparatus a photosensitive web may be provided with two types of perforations, one comprising a series of closely spaced perforations for providing timing signals and the other identifying frame locations on the web for use in timing or triggering flash exposures and other operations. See U.S. Pat. No. 4,025,186 to Hunt, Jr. et al. A problem with the apparatus described in this patent is that a mechanical arm is used to detect a perforation to time or synchronize the exposure operation. While the advantage of the arm is that it is successful in signaling that a perforation is present, it suffers from providing some inaccuracy as to the precise location of the perforation.
While photoelectric perforation detectors are well known, see for example, U.S. Pat. No. 3,936,650, problems arise in using such detectors with regard to determining perforation location with a high degree of precision. Also when used in an environment such as copiers described above, they are subject to contamination by toner dust. The dust settles on the components of the sensor system reducing readings to levels where random electrical noise may trigger the system to falsely indicate a perforation is present.
It is an object of the invention to provide an improved photoelectric sensor for sensing the location of an indicium such as a perforation and overcoming the deficiencies of the sensors known in the prior art.